| // Copyright 2011 The Chromium Authors |
| // Use of this source code is governed by a BSD-style license that can be |
| // found in the LICENSE file. |
| |
| #ifndef BASE_TEMPLATE_UTIL_H_ |
| #define BASE_TEMPLATE_UTIL_H_ |
| |
| #include <stddef.h> |
| |
| #include <iosfwd> |
| #include <iterator> |
| #include <type_traits> |
| #include <utility> |
| |
| #include "base/compiler_specific.h" |
| |
| namespace base { |
| |
| namespace internal { |
| |
| // Used to detech whether the given type is an iterator. This is normally used |
| // with std::enable_if to provide disambiguation for functions that take |
| // templatzed iterators as input. |
| template <typename T, typename = void> |
| struct is_iterator : std::false_type {}; |
| |
| template <typename T> |
| struct is_iterator< |
| T, |
| std::void_t<typename std::iterator_traits<T>::iterator_category>> |
| : std::true_type {}; |
| |
| // Helper to express preferences in an overload set. If more than one overload |
| // are available for a given set of parameters the overload with the higher |
| // priority will be chosen. |
| template <size_t I> |
| struct priority_tag : priority_tag<I - 1> {}; |
| |
| template <> |
| struct priority_tag<0> {}; |
| |
| } // namespace internal |
| |
| namespace internal { |
| |
| // The indirection with std::is_enum<T> is required, because instantiating |
| // std::underlying_type_t<T> when T is not an enum is UB prior to C++20. |
| template <typename T, bool = std::is_enum<T>::value> |
| struct IsScopedEnumImpl : std::false_type {}; |
| |
| template <typename T> |
| struct IsScopedEnumImpl<T, /*std::is_enum<T>::value=*/true> |
| : std::negation<std::is_convertible<T, std::underlying_type_t<T>>> {}; |
| |
| } // namespace internal |
| |
| // Implementation of C++23's std::is_scoped_enum |
| // |
| // Reference: https://en.cppreference.com/w/cpp/types/is_scoped_enum |
| template <typename T> |
| struct is_scoped_enum : internal::IsScopedEnumImpl<T> {}; |
| |
| // Implementation of C++20's std::remove_cvref. |
| // |
| // References: |
| // - https://en.cppreference.com/w/cpp/types/remove_cvref |
| // - https://wg21.link/meta.trans.other#lib:remove_cvref |
| template <typename T> |
| struct remove_cvref { |
| using type = std::remove_cv_t<std::remove_reference_t<T>>; |
| }; |
| |
| // Implementation of C++20's std::remove_cvref_t. |
| // |
| // References: |
| // - https://en.cppreference.com/w/cpp/types/remove_cvref |
| // - https://wg21.link/meta.type.synop#lib:remove_cvref_t |
| template <typename T> |
| using remove_cvref_t = typename remove_cvref<T>::type; |
| |
| // Simplified implementation of C++20's std::iter_value_t. |
| // As opposed to std::iter_value_t, this implementation does not restrict |
| // the type of `Iter` and does not consider specializations of |
| // `indirectly_readable_traits`. |
| // |
| // Reference: https://wg21.link/readable.traits#2 |
| template <typename Iter> |
| struct IterValueImpl { |
| using value_type = typename std::iterator_traits<Iter>::value_type; |
| }; |
| |
| template <typename T, bool Cond = false> |
| struct IterValuePointerImpl { |
| // The `iterator_traits<T*>::value_type` member is not defined if T is not an |
| // object in C++20. |
| }; |
| template <typename T> |
| struct IterValuePointerImpl<T*, true> { |
| using value_type = typename std::iterator_traits<T*>::value_type; |
| }; |
| |
| template <typename T> |
| struct IterValueImpl<T*> { |
| using value_type = |
| typename IterValuePointerImpl<T*, std::is_object_v<T>>::value_type; |
| }; |
| |
| template <typename Iter> |
| using iter_value_t = typename IterValueImpl<remove_cvref_t<Iter>>::value_type; |
| |
| // Simplified implementation of C++20's std::iter_reference_t. |
| // As opposed to std::iter_reference_t, this implementation does not restrict |
| // the type of `Iter`. |
| // |
| // Reference: https://wg21.link/iterator.synopsis#:~:text=iter_reference_t |
| template <typename Iter> |
| using iter_reference_t = decltype(*std::declval<Iter&>()); |
| |
| // Simplified implementation of C++20's std::indirect_result_t. As opposed to |
| // std::indirect_result_t, this implementation does not restrict the type of |
| // `Func` and `Iters`. |
| // |
| // Reference: https://wg21.link/iterator.synopsis#:~:text=indirect_result_t |
| template <typename Func, typename... Iters> |
| using indirect_result_t = |
| std::invoke_result_t<Func, iter_reference_t<Iters>...>; |
| |
| // Simplified implementation of C++20's std::projected. As opposed to |
| // std::projected, this implementation does not explicitly restrict the type of |
| // `Iter` and `Proj`, but rather does so implicitly by requiring |
| // `indirect_result_t<Proj, Iter>` is a valid type. This is required for SFINAE |
| // friendliness. |
| // |
| // Reference: https://wg21.link/projected |
| template <typename Iter, |
| typename Proj, |
| typename IndirectResultT = indirect_result_t<Proj, Iter>> |
| struct projected { |
| using value_type = remove_cvref_t<IndirectResultT>; |
| |
| IndirectResultT operator*() const; // not defined |
| }; |
| |
| } // namespace base |
| |
| #endif // BASE_TEMPLATE_UTIL_H_ |